1. Field of the Invention
The present invention relates to a retainer for retaining a rolling member within a ball bearing race. More particularly, this invention relates to a retainer with a rotationally symmetric pocket for which it is easy to manufacture a molding tool.
2. Description of the Prior Art
In bearings using rolling members, the escaping of the rolling members and the rubbing friction due to contact between the adjacent rolling members is prevented by using a retainer. The use of the retainer allows the rolling members to rotate within the race of the bearing and at the same time keeps them separated by a specified fixed distance.
FIG. 10 shows a conventional retainer 1 for use in a ball bearing. The retainer 1 is an annular member with a plurality of pockets 3 (only one pocket is shown in FIG. 10), which house balls 2. The balls 2 are packed into a race between an inner ring and an outer ring of the ball bearing (not shown). The balls 2 rotate in the race of the ball bearing. An inside surface 4A of the pocket 3 is of a spherical shape that matches the curvature of the surface of the balls 2. Generally, an inner diameter D1 of the pockets 3 is set to be slightly larger than the diameter D of the balls 2, for example, D1=1.03D. Since D1 is larger than D a gap C is formed between the balls 2 and the inside surface of the pockets 3. This gap C can hold a lubricant.
Use of the retainer 1 prevents contacts between the adjacent balls 2, and enables smooth rotation of the balls 2. As a result the frictional torque and frictional heat is suppressed and the inner and outer rings rotate smoothly relative to one another.
When the conventional retainer 1 is used, the gap C is formed between the balls 2 and the pockets 3. The gap C holds the lubricant. But when the rotational speed exceeds 10,000 rpm, irregular fluctuations occur in the gap C due to, for example, self-excited vibration of the balls 2. The irregular fluctuations in the gap C results in a pumping action which in turn causes fluctuation in the lubricant's pressure and fluctuation of shear resistance due to the lubricant's viscosity. As a result, the rotation of the ball bearing becomes unstable, the non-repeating run out (NRRO) increases, vibration and noise increase and lubricant leaks from the gap C.
The greater densification of the recording media and the increases in rotary speeds of the drive devices such as computer hard disks, magnetic disks and optical disks have created a demand for spindle motor bearings with high rotational accuracy, low friction, low noise and long life to drive these recording disks.
One way of reducing the vibrations is by making the gap C smaller. But when the gap C is made smaller, the contact surface between the balls 2 and the inside surface 4A of the pockets 3 increases. The increase in contact surface reduces the amount of lubricant held in the gap C and increases the rotary torque.
The above problem is solved by U.S. Pat. No. 4,225,199 and Japanese Utility Model S.57-87827 by making the inside surface 4A of the retainer pockets 3 of multi-faceted shape. The multi-faceted shape provides a fixed gap between the pocket inside surface 4A and the balls 2 and point contact between the balls 2 and the retainer 1, thereby, increasing lubricant retention and resulting in smooth rotation.
However, since the pocket inside surface 4A disclosed by the above mentioned prior art references is multifaceted in shape, the mold used to form the retainer is complex in shape. Since the manufacturing steps, such as cutting, grinding and other machining are more complicated for a complex shape, the mold is costlier to manufacture. Therefore, there is a need to provide a ball bearing retainer which will increase lubrication, permit smooth rotation by reducing vibration and noise and can be easy and cost effective to manufacture.